This paper presents a numerical study of dispersion and flammable volume of hydrogen in enclosures using a simple analytical method and a computational fluid dynamics (CFD) code. In the analytical method, the interface height and hydrogen volume fraction of the upper layer are obtained based on mass and buoyancy conservation while the centreline hydrogen volume fraction is derived from similarity solutions for buoyant jets. The two methods (CFD and analytical) are used to simulate an experiment conducted by INERIS, consisting of a 1 g/s hydrogen release for 240 s through a 20 mm diameter orifice into an enclosure. It is found that the predicted centreline hydrogen concentration by both methods agrees with each other and is also in good agreement with the experiment. There are however differences in the calculated total flammable volume because the analytical method does not consider local mixing and diffusion in the upper layer which is assumed uniformly well mixed. The CFD model, in comparison, incorporates the diffusion and stratification phenomena in the upper layer during the mixing stage. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.